Doubling the frame rate of a television signal provides a desirable reduction in "flicker" when the signal is displayed. One known method of doubling the frame repetition rate is the "repeat field" method in which each field received is displayed two times. For example, for the received fields A, B and C in a repeat field system, each field would be stored in a memory at a given write clock rate and recovered two times from the memory with a higher read clock rate to provide a display field sequence of A-A-B-B-C-C. Repeating each field once thus doubles the frame rate, repeating twice triples the frame rate, etc.
Another known form of frame rate doubling system employs interpolation to obtain extra fields for display rather than repeating fields. In one such system a second reproduced field (A') is interpolated from the first field that is transmitted (A), a third reproduced field (B') is interpolated from the second field transmitted (B) and a fourth reproduced field (B) is repeated from the second field transmitted (B) to provide a double field rate output signal for display (A-A'-B'-B). However, it has been found that undesirable disturbances may occur in such systems such as line flickering or motion artifacts.
A doubling of the frame repetition rate using a symmetrical double median filtering is achieved in a system described in DE-A-4 220 662. In this system picture elements from lines between the lines of the input fields are calculated using a vertical temporal median filter. Then, progressive fields having double the frame repetition rate are generated from the progressive interpolated fields having e.g., a 50 Hz frame repetition rate with the aid of the formation of a temporal average value. However, it is herein recognized that if these output fields are reproduced with an artificial interlace, then 25 Hz disturbances (e.g., the input signal frame rate) can occur. It is further herein recognized that the problem of such disturbences results, at least in part, because, on each occasion, two successive 100 Hz fields only consist of non-interpolated picture elements.